Post-collisional extension of the South Altun subduction-collision belt, northern Tibetan Plateau: Insight from phase equilibria modeling and zircon geochronology of pelitic migmatites

Abstract

• Continental collision led to the burial of pelitic rocks in the South Altun. • Pelitic migmatites were formed under the conditions of ∼850–870 °C and 0.6–0.7 GPa. • High- T/P type metamorphism and migmatisation lasted for ∼10 Myr (457–447 Ma) • Continental subduction to post-collisional extension took place in ∼50 Myr. The South Altun subduction-collision belt in the northern Tibetan Plateau is one of the rare examples of ultradeep subduction (>350 km). In addition to subduction-related low-temperature/pressure ( T/P ) type metamorphic rocks, the belt also exposes high- T/P metamorphic rocks with well-developed migmatitic structures, which provide insight into the post-collisional process. Phase equilibria modeling of the Tula pelitic migmatites indicates peak conditions of ∼850–870 °C and 0.6–0.7 GPa for the high- T/P type metamorphism and migmatism. Phase equilibria modeling predicts this high temperature could result in the production of 10–20 vol% melt in metapelites, producing stromatic migmatites observed in multiple outcrops. Based on results of zircon U-Pb geochronology of granulite-facies migmatites, we interpret the supra-solidus conditions to last for ∼10 Myr (457–447 Ma), corresponding to short-lived granulite-facies metamorphism in post-collisional extension. The deposition of the protolith is inferred to occur immediately before the metamorphism on active continental arcs based on the youngest detrital and oldest metamorphic ages. Combined with studies on subducted-related metamorphic rocks, these findings suggest that the South Altun belt experienced the continental subduction (early collision) to the post-collisional extension within a period of ∼50 Myr. At the same time, the thermal regime of this orogenic belt evolved from low- T/P to high- T/P whereas the high- T/P regime predominates the orogenic belt towards the end of orogenic evolution, as evidenced by regional high- T/P type metamorphic rocks and overprinting on the ancient low- T/P regime of exhumed subduction-related rocks.